This application primarily addresses broad Challenge Area (04) Clinical Research and specific Challenge Topic, 04-HL-103, "Assess the role of leukocyte interaction with platelets, erythrocytes, and endothelium in the pathogenesis of heart, lung, and blood diseases". It also addresses broad Challenge Area (06) Enabling Techniques and specific Challenge Topic 06- HL-105, "Develop transgenic animal models that are informative for understanding chronic inflammation in humans". Platelet disorder, directly or indirectly, affects all human inflammatory diseases. One of the major pathways for platelet activation is through the interaction between platelet glycoprotein Ib- IX-V complex and the endothelial cell-derived von Willebrand factor (VWF), which is tightly regulated physiologically. A great deal has been learnt on the downstream signaling of GPIb-IX- V complex. However, little is known on how GPIb-IX-V macromolecular complex is assembled and what are the roles of molecular chaperones in this process. The deficiency in this area is surprising in light of the fact that the defect of platelet GPIb-IX-V complex causes clinical macrothrombocytopenia such as Bernard-Soulier syndrome (BSS), and the GPIb-IX-V complex has always been the attractive pharmacological target for platelet disorders. gp96 (grp94, HSP90b) is a paralogue of HSP90 in the endoplasmic reticulum (ER) where the assembly of GPIb-IX-V complex occurs. By generating conditional gp96 null mice, we have found that gp96 is an essential chaperone for multiple integrins and Toll-like receptors (TLRs). Unexpectedly, we have discovered that gp96 ablation in murine hematopoietic system did not negatively affect megakaryopoiesis but resulted in prolonged bleeding time, thrombocytopenia and giant platelet disorder that are clinically indistinguishable from human BSS. Moreover, loss of gp96 causes concurrent reduction of cell surface GPIb but not aIIb[unreadable]3 integrin (GPIIb/IIIa). A variety of small molecule inhibitors are already available for targeting HSP90, including gp96. Overall, we believe that the successful execution of this study shall lead to a new understanding of the pathogenesis of BSS, and the "identification of the key points controlling" platelet activation, which "may lead to new pharmaceutical interventions (i.e., anti-platelet agents) for both thrombosis and inflammation", as stipulated in 04-HL-103. This project also will provide "targeted research over short period of time", "lead(ing) to development of new animal models for chronic inflammation that are relevant to human pathology", as called upon by 06-HL-105. This project addresses the fundamental mechanism of Bernard-Soulier syndrome (BSS), a bleeding disorder due to problems in several molecules on platelets and their progenitors. Proteins must adopt a proper 3-dimensional shape in order to function. Such a process is often catalyzed by molecules referred to as heat-shock proteins (HSPs). We have data to implicate for the first time that loss of one HSP called gp96 can cause BSS. We aim to discover the molecular details of how gp96 facilitates the maturation of BSS proteins. Successful execution of this study may lead to new understanding of the causes of BSS and potential novel treatments of BSS and other platelet disorders. PUBLIC HEALTH RELEVANCE: This project addresses the fundamental mechanism of Bernard-Soulier syndrome (BSS), a bleeding disorder due to problems in several molecules on platelets and their progenitors. Proteins must adopt a proper 3-dimensional shape in order to function. Such a process is often catalyzed by molecules referred to as heat-shock proteins (HSPs). We have data to implicate for the first time that loss of one HSP called gp96 can cause BSS. We aim to discover the molecular details of how gp96 facilitates the maturation of BSS proteins. Successful execution of this study may lead to new understanding of the causes of BSS and potential novel treatments of BSS and other platelet disorders.